1. Field of the Invention
This invention relates to a method and apparatus for deposition of solid electrolyte materials on porous substrates suitable for use as electrodes in fuel cells. More particularly, this invention relates to a method and apparatus for fabrication of electrolyte films by electrophoretic deposition (EPD) on porous substrates suitable for use as electrodes in high temperature solid oxide fuel cells.
2. Description of Prior Art
In recent years, noteworthy technological advancements have been made to expedite the commercialization of high temperature solid oxide fuel cells (SOFCs)--a non-conventional device for producing electrical energy from fossil fuels. Globally, the potential of fuel cell technology has been well-recognized, mainly due to its unprecedented high electrical power efficiency, reliability, modularity, fuel adaptability, environmentally friendly end-products (very low levels of NO.sub.x and SO.sub.x emissions) and noise-free operation during electricity production. See N. Q. Minh and T. Takahashi, Science and Technology of Ceramic Fuel Cells, Elsevier, N.Y., 1995; S. C. Singhal, Proceedings of the Fifth International Symposiums on Solid Oxide Fuel Cells, Eds. U. Stimming et al. Electrochemical Soc., Pennington, p. 37, 1997; N. Q. Minh, J. Am. Ceram. Soc., 76 (1993) 563; S. P. S. Badwal et al, Ceramic International, 22 (1996) 257 and Materials Forum, 21 (1997) 187; and B. C. H. Steele, Ceramic International, 19 (1993) 269. Among the different SOFC designs under development, (See "Proceedings of the First to Fourth International Symposiums on Solid Oxide Fuel Cells," Electrochemical Soc., Pennington, 1989, '91, '93, and '95 respectively.; and Fuel Cell Seminar Abstracts, Palm Springs, Calif., USA, November, 1998.) the sealless tubular design is recognized as the design closest to commercialization. As shown in FIG. 1, the design consists of a porous inner air electrode support cathode tube 10, made of a doped lanthanum manganite (LaMnO.sub.3) material having 30-35% porosity, an intermediate zirconia electrolyte layer (a gas-tight layer of ZrO.sub.2 -8 mol % Y.sub.2 O.sub.3) 11, a porous fuel electrode 12 on the outer surface made of a nickel-zirconia cermet material and an interconnection (gas-tight) 13, made of a doped LaCrO.sub.3 material. The main challenge presented by the tubular design is to reduce the cost of the cell without negatively impacting other properties, for example, cell performance, resistance to thermal cycles, total lifetime, etc.
At the present time, the expensive electrochemical vapor deposition (EVD) technique, which is a major factor affecting costs is used for depositing the gas-tight electrolyte. As a result, there is a worldwide effort underway to replace electrochemical vapor deposition with less expensive deposition techniques.
Several non-EVD fabrication approaches to depositing electrolyte films over porous air electrode supported (AES) cathode tubes have been reported in the literature in an attempt to replace the expensive EVD process. With these approaches, most of the deposited particulate coatings of zirconia must be sintered to achieve full density. These approaches include low pressure plasma spray, N. Hisatome et al, Proceedings of the Fifth International Symposiums on Solid Oxide Fuel Cells, Eds. U. Stimming et al., Electrochemical Soc., Pennington, p. 180, 1997 and Fuel Cell Seminar Abstracts, Palm Springs, Calif., USA, November, p.28, 1998., vacuum plasma spray, G. Schiller et al., Proceedings of the Fifth International Symposiums on Solid Oxide Fuel Cells, Eds. U. Stimming et al. Electrochemical Soc., Pennington, p. 635, 1997 and Fuel Cell Seminar Abstracts, Palm Springs, Calif., USA, November, p. 515, 1998., and colloidal processes such as slurry-based dip or spray coating, S. Swartz et al., Fuel Cell Seminar Abstracts, Palm Springs, Calif., USA, November, p. 72, 1998. The plasma and vacuum spray techniques have high equipment and accessory costs. The colloidal approaches are much cheaper. However, dip and spray coating have poor reproducibility and control over the thickness of the deposited film.
Electrophoretic deposition is a colloidal process in which charged particles dispersed in a stable suspension are driven to move towards an oppositely charge electrode, upon which they ultimately deposit, to build up a particulate coating. The technique is extremely inexpensive and has been used for many years to fabricate green ceramic bodies and coatings with different shapes for applications ranging from ceramic/ceramic and metal/ceramic composites to thin/thick film coatings for electronic devices (P. Sarkar and P. S. Nicholson. J. Am. Ceram. Soc., 79, 1987 (1996); L. Vandeperre and O. Van Der Biest. Key Engineering Materials. 132-136, 2013 (1997); and N. Koura, A. Taniguchi, H. Shoji, S. Ito and Y. Takayama. J. Ceram. Soc. Jpn., Int. Edition 104, 810 (1996)). This processing technique is especially useful for the preparation of uniform particulate coatings with high green densities (50-60%) and controlled thickness. The present investigation explores EPD as a processing technique for producing high density films for SOFC electrolytes on AES porous doped LaMnO.sub.3 (LDM) cathode tubes. The films are produced in the unfired state, and they are later fired to produce the full density and hermetic seal required of the electrolyte layer. To date, the primary reason EPD has not been used to produce zirconia electrolytes for solid oxide fuel cells is the lack of a viable EPD chemistry and process developed specifically for SOFC applications.
To our knowledge, the only published account of using EPD for the production of zirconia electrolytes for SOFC's was reported in 1993, T. Ishihara et al., Proceedings of 3.sup.rd Intl. Symposium on SOFC (p. 65, 1993); Proceedings. of 4.sup.th Intl. Symosium on SOFC (p. 334,1995); and J. Am. Ceram Soc., 79, 913 (1996). In these basic studies, zirconia electrolyte films were deposited on disc-shaped substrates of porous Sr-doped LaMnO.sub.3 (LSM) cathode and anode (Ni--YSZ) materials using at least 6 deposition steps, each followed by a sintering step. This method is not suitable for mass production for several reasons: 1) the suspension chemistry is subject to high variability (strongly dependent on iodine concentration in an acetylacetone solvent), 2) the principle has been demonstrated only on flat samples with small dimensions (20 mm discs, in the case of the LSM substrates), and 3) the technique of multiple deposition and sintering steps is complicated, time-consuming, and inordinately expensive. In addition, without the multiple deposition steps, a gas-tight electrolyte layer, and hence acceptable fuel cell performance, cannot be obtained.